1. Field of the Invention
This invention relates to a method for measuring and analyzing interference fringes using a hologram interferometer, which employs a holographic optical element, for obtaining the configuration of a surface of an object to be measured.
2. Description of the Prior Art
The interference method is a widely known technique for accurately measuring the configuration of a spherical or aspherical surface. The measurement of configuration by the interference method is directed to obtain the degree of displacement of the surface to be measured relative to the high precision flat reference surface (the interference standard) from interference fringes which occurred as a result of interference of beams reflected from each surface.
The foregoing interference method is advantageous in that the accuracy of configuration of all surfaces can be verified instantaneously without making contact with the surface. Among others, an interference method using a computer generated hologram is now attracting attention, because it allows special configurations, other than those for flat surfaces or aspherical surfaces, to be measured. This computer generated hologram is produced by the steps of exposing a glass substrate covered with a photoresist to light by scanning electron beams across the substrate, and developing this pattern, so that the pattern becomes visible.
As one example of such hologram interferometers using the forgoing holographic optical element, there is put forward a holographic optical element provided with a first pattern for diffracting a laser beam as a reference beam in a direction which is different from that of a surface to be measured and a second pattern for diffracting a laser beam as a measuring beam which is to be radiated to that surface, and this holographic optical element measures interference fringes occurring as a result of interference between the reference beam and a beam reflected from the surface to be measured via the holographic optical element having the foregoing two types of patterns. The use of this type of hologram interferometer renders the flat reference surface unnecessary.
As another type of interferometer, the Fizeau interferometer is also widely accepted wherein interference fringes resulting from optical interference which occurs in response to a distance between the flat reference surface and the object are measured. As a measuring technique which employs this Fizeau interferometer, a fringe scanning technique is commonly known, wherein interference fringes are analyzed on the basis of variations in brightness at a certain location in accordance with changes of a relative distance between the object and the flat reference surface. The wide use of this technique owes to the fact that very accurate results about the configuration of the surface can be obtained.
Thus, since the hologram interferometer equipped with the holographic optical element having the foregoing two patterns does not have a flat reference surface, it is difficult to apply the fringe scanning technique, which successively measures interference fringes by moving the flat reference surface so that the distance between the flat reference surface and the object can be changed, to the measurement using the hologram interferometer.
In other words, according to the fringe scanning technique adopted in general Fizeau interferometers, the phase of interference fringes is changed by nominally displacing the position of the flat reference surface toward the optic axis in such a manner that a difference in an optical path length, between the reference beam reflected from a reference plane of the flat reference surface and an object beam reflected from a surface to be measured after having passed through the flat reference surface, is successively changed.
If the holographic optical element is moved along the optic axis by applying such a fringe scanning technique to the hologram interferometer, it is impossible to change the phase of interference fringes, and hence it is impossible to measure and analyze interference fringes by means of the fringe scanning technique.